This invention relates generally to the field of group III-V (group III-nitride) semiconductors, and in particular to a method of growing Gallium nitride (GaN) epilayers for use in electronic devices.
Gallium nitride has strong potential for applications in high power, high frequency electronic devices due to its high electron saturation velocity, high critical field, and high stability. Moreover, the large band discontinuity and close lattice match between GaN and its ternary alloy AlGaN make it possible to form a high quality AlGaN/GaN hetero-interface that can confine an unusually high density of electrons in a two-dimensional electron gas. The high solubility of the AlGaN alloy and large piezoelectric field in the strained heterostructure allow extensive and sophisticated band structure engineering and tailoring for specific and optimal device performance. Modulation-doped field effect transistors (MODFETs) based on the AlGaN/GaN heterostructures; have been demonstrated to show very promising high frequency, high power, and high temperature performance.
Two growth techniques, namely metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), have been successful in growing these high quality MODFET structures. Though both techniques have yielded devices with similar performance, the MOCVD grown AlGaN/GaN heterostructures have shown a higher mobility of the two-dimensional electron gas, which is typically about 1000-1200 cm2/Vs at room temperature for growth on sapphire substrates and up to about 2000 cm2/Vs for growth on SiC substrates.
One of the key requirements in the growth of GaN MODFETs is the growth of a thick semi-insulating GaN layer. The GaN layer has to be thick enough (usually greater than 1 micron) in order for the AlGaN/GaN interface to avoid the highly defective region near the GaN/substrate interface caused by the lattice mismatch. Meanwhile, the GaN layer needs to be highly resistive. A conducting GaN buffer layer will cause leakage current and degradation of r.f. performance of the device.
However, little is known about growing the highly resistive GaN layer. Even less is understood about the physical origin of the semi-insulating property of the GaN layers grown for the MODFETs.
Using an MSE deposited AMN nucleation layer, it is possible to grow high quality Si-doped GaN bulk layers with record high mobilities (up to 560 cm2/Vs for a carrier density of 1.4xc3x971017 cmxe2x88x923 at room temperature) for the MBE technique. It is however surprisingly difficult to grow highly resistive GaN by ammonia-MBE since the undoped layers tend to acquire an autodoping concentration typically in the range of 1016 to 1018 cmxe2x88x923, depending on the growth conditions. This autodoping can be eliminated by using certain growth conditions, such as a lower growth temperature and/or low III to V ratio, which could introduce acceptor-type defects to compensate for the n-type carriers. However, this approach requires using non-optimal growth conditions and suffers from poor reproducibility. The applicants have been unable to produce truly high resistivity undoped GaN layers by varying growth temperature and III/JV ratio in such a system. The highest resistivity achieved was about 500 xcexa9-cm and was not reproducible. An object of the invention is to provide a method of growing semi-insulating GaN.
According to the present invention there is provided a method of making a device with a semi-insulating GaN layer, comprising growing said semi-insulating gallium nitride layer using molecular beam epitaxy (MBE) in the presence of a nitrogen source and carbon dopant.
MBE is a technique for laying down layers of these materials with atomic thicknesses on to substrates. This is done by creating a molecular beam of a material, gallium in this case, which impinges on to the substrate and then reacting with the ammonia to create epitaxial layers of GaN.
Carbon is an acceptor in GaN when substituting the nitrogen site. It is the only group IV element that has a small enough atomic radius to enter the nitrogen site. Compared with other group II acceptors such as Mg and Ca, carbon is much less volatile and therefore not prone to causing a residual memory effect in the growth system. It is therefore a good compensating dopant for achieving semi-insulating GaN.
In a preferred embodiment, methane gas injected into a saddle field ion gun is used for the carbon dopant source. Other plasma sources such RF and ECR can be employed.
Control of the doping level is achieved by adjusting parameters of the ion source, such as the ion kinetic energy (anode voltage) and the ion current. Semi-insulating C-doped GaN layers with resistivities greater than 108 xcexa9-cm have been obtained with high reproducibility and reliability. Using the C-doped semi-insulating GaN as a thick buffer layer, high quality AlGaN/GaN heterostructures which exhibited a two-dimensional electron gas with high mobility have successfully been grown. The highest room temperature mobility achieved so far is 1210 cm2/Vs with a corresponding 77K mobility of 5660 cm2/Vs. These mobility values are believed to be the highest reported for a two-dimensional electron gas grown by MBE, and are comparable to the best mobilities reported for AlGaN/GaN heterostructures grown by MOCVD on sapphire substrates and can be compared to values of 1150 cm2/Vs and 3440 cm2/Vs at 300K and 77K recently reported for N-face structures and 1190 cm2/Vs at 300K for the Ga-face structures grown by plasma source MBE. More importantly, the high mobility heterostructures can be grown on a truly insulating GaN base, allowing practical device fabrication using these structures.
The invention also provides an apparatus for growing a semi-insulating GaN layer, comprising a vacuum chamber for supporting a substrate, a gallium beam source directed at said substrate, a source of nitrogen, and a source of carbon ions directed at said substrate.